Multistage Evolution and Inheritance in a Magma‐Poor Conjugate Margin: Insights From Serpentinized Peridotites of the Newfoundland Margin (ODP Hole 1277A)

The nature of mantle sections exhumed at magma-poor rifted margins remains poorly constrained, largely because direct sampling of mantle in modern passive margins is exceptionally rare. The Iberia–Newfoundland system is unique in providing drilled mantle rocks on both conjugate margins, offering an unprecedented glimpse into mantle heterogeneity across an entire rift system. Here we present new in situ major and trace element data from highly serpentinized peridotites recovered at ODP Hole 1277A (Newfoundland margin). The samples display mylonitic textures and are strongly depleted in clinopyroxene (≤2 vol.%). Despite pervasive serpentinization, relict mantle minerals retain evidence of high melt extraction degrees, as indicated by Al2O3-poor orthopyroxene (2.36–3.19 wt.%) and orthopyroxene trace element systematics. Geochemical modeling of orthopyroxene REE compositions indicates ≥14% melt extraction initiated in the garnet stability field, likely predating the Jurassic rifting. Following depletion, the mantle was locally overprinted by melt-rock interaction, as highlighted by high and variable spinel Cr#, REE disequilibrium in coexisting pyroxenes, and clinopyroxene with atypical V-shaped REE profiles (LaN/SmN = 4.3). During Jurassic rifting, the Newfoundland margin mantle section was reactivated and deformed within lithospheric-scale shear zones, leading to mylonitization and exhumation. The refractory nature of the mantle likely promoted limited melt extraction and retention of these transient melts within the lithosphere, now preserved as spinel-pyroxene symplectites. Subsequent cooling enabled hydration at ∼500–600°C and ultimately pervasive serpentinization. These results highlight that the pre-rift evolution of the lithospheric mantle exerts a primary control on the evolution of magma-poor rifted margins.